1. Field of the Invention
The present invention generally relates to radio communications systems and particularly relates to radio communications systems, base station and user apparatuses, and methods.
2. Description of the Related Art
In a related-art time division multiple access (TDMA) mobile communications system, mutually different frequencies are used in neighboring cells. A certain group of frequencies is used for a set of multiple cells, while the same group of frequencies is used for a different set of cells that are geographically distant. Such a scheme is preferable from a viewpoint of reducing other-cell interference, but is not high in frequency utilization efficiency.
On the other hand, in a W-CDMA (Wideband Code Division Multiple Access) scheme such as IMT-2000, users are distinguished with spread codes, so that the same frequency is used at all cells. Such a technique is called “one-cell frequency repetition”. The one-cell frequency repetition significantly increases frequency utilization efficiency and system capacity. It is anticipated that realization of the one-cell frequency repetition is also going to be needed even in future mobile communications systems such as IMT-Advanced (also called LTE-Advanced in 3GPP (3rd Generation Partnership Project), which is to be studied in the future.
However, there is a concern for a problem that, as the same frequency is used among neighboring cells, interference level (intercell interference) is likely to become large, particularly at a cell edge.
For intracell interference, in the W-CDMA scheme, OVSF (orthogonal variable spreading factor) code is used for downlink in order to realize intracell orthogonalization (i.e., interuser orthogonalization). However, in the W-CDMA scheme, orthogonalization cannot be realized in a multi-path environment, so that it is not orthogonal for uplink. In E-UTRA (Evolved UMTS Terrestrial Radio Access) scheme, orthogonalization is realized by performing frequency scheduling at a base station for both uplink and downlink.
On the other hand, for the intercell interference, in E-UTRA scheme, a technique called intercell interference coordination (ICIC) is used. In this technique, in addition to the fact that a frequency which is common to all cells is used, a frequency which differs from cell to cell is used at the cell edge (see Non-patent document 1).
FIG. 1 is a diagram illustrating the intercell interference coordination, which is adopted in the E-UTRA scheme. In the intercell interference coordination, radio resources are divided into radio resources R1-R3 which are usable only at corresponding base stations, and radio resources R4-R8 which are usable in a manner common to all of the base stations. The radio resources R1-R3 are frequencies allocated to the user at the cell edge, while radio resources R4-R8 are frequencies allocated to a user of a region other than the cell edge (for example, a user located in the vicinity of the base station, etc.) The radio resource R1 is used for a user belonging to the cell edge of a base station BS1, while it is not used for a neighboring base station. Similarly, the radio resource R2 is used for a user belonging to the cell edge of a base station BS2, while it is not used for a neighboring base station. The radio resource R3 is used for a user belonging to the cell edge of a base station BS3, while it is not used for a neighboring base station. Thus, the user of a cell edge of the base stations BS1-BS3 may conduct communications with small interference.
Information on a radio resource usable only at one of the base stations is shared among the base stations by using a backhaul (core network) and/or radio control signal. Here, such intercell interference coordination is called distributed autonomous intercell interference coordination.
Moreover, in order to reduce the intercell interference, a technique is also being studied in which one base station collectively manages radio resources of neighboring base stations. Such an intercell interference technique is used for decreasing intersector interference when a cell of a base station is divided into multiple sectors, or for decreasing intercell interference when there is a protruding cell.
FIG. 2 is a diagram illustrating intercell interference coordination for one base station BS1 to collectively manage radio resources of neighboring base stations BS2 and BS3. The base station BS1, which is connected to the neighboring base stations BS2 and BS3 via an optical fiber, etc., collectively allocates radio resources used by the neighboring base stations BS2 and BS3. For example, the base station BS1 allocates radio resources R1, R2, and R5 to users within the base station BS1, allocates radio resources R3 and R4 to users within the base station BS2, and allocates radio resources R6-R8 to users within the base station BS3.
In this way, the base station BS1 may allocate radio resources without causing interference. Here, such intercell interference coordination is called central control intercell interference coordination. Moreover, a base station which collectively manages radio resources is called a control base station or a central control base station, while abase station which manages radio resources with the control base station is called a remote base station.    Non-patent Document 1: 3GPP R1-060670, Siemens, “Interference Mitigation by Partial Frequency Reuse”    Non-Patent Document 2: 3GPP TS36.211